The present invention relates to a telephoto system for use with 35 mm single-lens reflex cameras. More particularly, the present invention relates to a high-speed telephoto lens system having an F number of 2.0 and a focal length of about 100 mm.
Auto focus single-lens reflex cameras which perform automatic focusing with a motor-driven lens system have seen a rapid increase in popularity. In response to this market trend, there has been a growing need to reduce the weight of lens groups movable during focusing in order to accomplish faster focusing with photographic lenses.
Two types of lens systems have conventionally been used as telephoto lenses having a focal length of about 100 mm, i.e., a modified Gauss type as described in JP-A-53-10425 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-57-40218, JP-A-58-21221, JP-A-59-48723 and JP-A-62-244010, and an Ernostar type as described in JP-A-50-62630, JP-A-51-62037, JP-A-51-77226, JP-A-53-133029, JP-A-54-50321, JP-A-55-124115, JP-A-58-126512, JP-A-59-65820, and JP-A-59-65821. While numerous improvements have been made in these types of lens systems, the examples mentioned above are the closer in design to the lens system of the present invention.
Lens system of a modified Gauss type are advantageous for the purpose of increasing the effective aperture ratio, but they suffer from the disadvantage that the lens closer to the image than the diaphragm stop has a tendency to increase in size. Further, these lens systems are made of glass having a comparatively high refractive index, but the high index glass has such a high specific gravity that it is not suitable from the standpoint of reducing the lens weight. In contrast, lens systems of an Ernostar type are advantageous in that they are light in weight since the rear group can be made of a simpler construction than lenses of a modified Gauss type. However, it has been difficult to attain a large effective aperture ratio on the order of 1:2.0 with these lens systems although they are capable of efficient compensation for aberrations up to the level of 1:2.8.